When circuit boards are inserted into a live (e.g.,−48V) backplane or when a new battery is applied to a system, a bypass (filtering) capacitor connected across the board's power module can draw huge transient currents (in-rush currents) as it charges up. These transient currents can cause permanent damage to the board's components and cause glitches on the system power supply. There is a voltage step that occurs at the input of the system when power is initially applied or a new battery is connected. This voltage step may be tens of volts. If the voltage step to the load is instantaneous, the step can be potentially damaging to the load. Therefore, a ramping of the voltage to the load is desirable and is accomplished using a hot swap system.
A typical hot swap system allows a load (including any bypass capacitor across the voltage terminals) to be connected to a power supply so that the in-rush current into the load is limited to a particular maximum value to protect the system from over-currents and over-heating. The hot swap system typically includes a pass MOSFET in series between the power supply and the load, where the MOSFET is controlled to limit the current supplied by the power supply to a predetermined value. The current is typically sensed by detecting the voltage drop across a low value sense resistor in series with the MOSFET.
A conventional hot swap controller simply limits the current through the MOSFET to a predetermined value somewhere above a worst case acceptable load condition. Therefore, when the power supply is initially connected to the load (including a bypass capacitor) or if there is any other input voltage step, the charging ramp of the bypass capacitor will be limited (clamped). However, such a current limit level is typically set based on the normal operating conditions rather than for start-up or voltage step conditions. Thus, the current limit level is not optimized for the start-up or voltage step conditions. This can result in an unduly long charging time (if the current limit level is set too low) and/or an over-heating problem if the limited current occurs over an extended ramp time.
Another known technique is to use a capacitor (not the load's bypass capacitor) in the hot swap controller that charges at a controlled rate and is used to ramp up the gate voltage of the pass MOSFET to ramp up and control the in-rush current. However, there is no adaptive optimizing of the in-rush current for the particular load.
If the in-rush current is too high, the MOSFET will overheat. Surprisingly, if the in-rush current is made too low, the load (including the bypass capacitor) will take a long time to be fully powered up, and the MOSFET will overheat due to the extended power dissipation time. A MOSFET must not exceed a specified peak temperature, which may be supplied by the manufacturer or determined by testing.
What is needed is a circuit, such as for hot swapping or other condition where there is an input voltage step, where the in-rush current through the pass MOSFET is adaptively and optimally controlled to achieve a specified or minimum peak temperature of the pass MOSFET. After the bypass capacitor is fully charged, a conventional current limit circuit may then be used for over-current protection during normal operation.